It is known that video signals can be obtained from images on color film and can be reproduced on a television receiver or a monitor television screen.
FIG. 3 is a block diagram of a prior art system used for the above purpose. Referring to FIG. 3, numeral 1 indicates a roll of negative color film, numeral 2 indicates a light source, numeral 3 indicates a color image pickup unit using vacuum tube or solid state image pickup devices, numerals 4 through 6 indicate R, G, and B color signals obtained from the color image pick up unit 3, numeral 7 indicates an inverting circuit, numerals 8 through 10 indicate gamma-corrected and inverted R, G, and B color signals, numeral 11 indicates an optional magnetic recording unit, numerals 12 through 14 indicate reproduced R, G, and B color signals from the recording unit 11, numeral 15 indicates a composite color video signal in NTSC system, and numeral 16 indicates an image display unit.
The inverting circuit 7 inverts and amplifies the respective R, G, and B signals in order to provide a signal representative of a printed positive image obtained from the negative color film 1. The inverting circuit 7 also has the function of gamma correction for the R, G, B signals to provide a normal contrast ratio in the reproduced image.
Heretofore, the gamma value Yp of the inverting circuit 7 has been constant, and since
(1) Gamma value Yn of a normally exposed color film negative is approximately 0.65, and
(2) Gamma value Yr of an image pickup tube is approximately 1, and
(3) Gamma value Ys of a cathode-ray tube is approximately 2.2,
the Yp is set at ##EQU1## so that the overall gamma value is 1.
However, as can be seen from the characteristic curve of a color negative film shown in FIG. 4, gamma value of color negative film is not constant but varies with the extent of exposure of the film. Referring to FIG. 4, the vertical axis is scaled for color negative film density (D) and the horizontal axis is scaled for the logarithm of exposure (log E). Yn is approximately 0.65 for normally exposed film lying within exposure area 17, approximately 0.4 for film lying within area 18, and is approximately 0.3 for under exposed film lying within area 19. Therefore, the system shown in FIG. 3 in which the gamma value of the inverting circuit 7 is fixed at Yp=approximately 0.7, is not effective in that the reproduced image is too soft and of low contrast if the color negative film has been under or over exposed.